In light of the problem of ozone layer depletion that is a focus of attention in recent years, the restrictions on CFCs (chlorofluorocarbons) and HCFCs (hydrochlorofluorocarbons) that are used as refrigerants in conventional refrigerating machines have become more stringent, and HFCs (hydrofluorocarbons) are being used as substitute refrigerants.
Mineral oils or hydrocarbon oils such as alkylbenzenes have been preferred for use as refrigerating machine oils when CFCs or HCFCs are the refrigerants, but since changing the refrigerant can cause the refrigerating machine oil used with it to exhibit unpredictable behavior in terms of its compatibility with the refrigerant, its lubricity, its viscosity when dissolved with refrigerants and its thermal and chemical stability, it has been necessary to develop different refrigerating machine oils for different refrigerants. Examples of refrigerating machine oils that have been developed for HFC refrigerants include polyalkylene glycols (see Patent document 1), esters (see Patent document 2), carbonic acid esters (see Patent document 3) and polyvinyl ethers (see Patent document 4).
HFC-134a is an HFC refrigerant that is routinely used as a refrigerant for automobile air conditioners, and while it has an ozone depletion potential (ODP) of zero, its high global warming potential (GWP) has led to its restriction in Europe. It has therefore become an urgent issue to develop refrigerants that can be used as substitutes for HFC-134a.
In light of this background it has been proposed to use, as substitute refrigerants for HFC-134a, unsaturated fluorinated hydrocarbon refrigerants which have very low ODP and GWP, are noncombustible and are comparable or superior to HFC-134a in terms of their thermodynamic properties, as a measure of refrigerant performance. There has also been proposed the use of refrigerant mixtures of unsaturated fluorinated hydrocarbons with saturated hydrofluorocarbons, C3-5 saturated hydrocarbons, dimethyl ether, carbon dioxide, bis(trifluoromethyl)sulfide or trifluoroiodomethane (see Patent document 5).
In addition, there have been proposed refrigerating machine oils that employ mineral oils, alkylbenzenes, poly α-olefins, polyalkyleneglycols, monoesters, diesters, polyol esters, phthalic acid esters, alkyl ethers, ketones, carbonic acid esters, polyvinyl ethers and the like, as refrigerating machine oils that can be used with unsaturated fluorinated hydrocarbon refrigerants or refrigerant mixtures of unsaturated fluorinated hydrocarbons and saturated hydrofluorocarbons, C3-5 saturated hydrocarbons, dimethyl ether, carbon dioxide, bis(trifluoromethyl)sulfide or trifluoroiodomethane (see Patent documents 5-7).